Prosthetic Sockets Have a Tendency to be Over-Designed

Maria Gerschutz, PhD, an applied research engineer for
Ohio Willow Wood, explored the effects of common over-design techniques in a
study, presented at the 2011 Academy Annual Meeting and Scientific Symposium.
According to Gerschutz, much of the variability in socket performance is
related to poor control processes that can lead to over-design.

“In my opinion,
computer aided design/computer aided manufacturing (CAD/CAM)
systems can significantly reduce the number of error variation in socket design
variations which will affect fit function,” she told O&P
Business News
. “In our study, we specifically focused on the use of
CAD/CAM systems, which eliminated some of the many variables that can result in
variation in socket performance.”

There are numerous variations of socket design due to
lack of standardization. Michael Haynes, MS, a design engineer for Ohio Willow
Wood, described over-design as the creation of a socket so strong that it
becomes too bulky for the patient. A patient needs his or her socket to be
strong and accurately measured to ensure comfortability, but because there is
no standard for design and strength of a prosthetic socket, each practitioner
essentially creates his or her own standard for performance.

Whatever the standard, each provider must account for
this variation and performance by building a socket that will be more robust
than they would be typically comfortable with if they could tightly control the
behavior of the final product, according to Gerschutz.

“There is no standard in a field where there are
hundred or thousands of shops developing their own processes,” Haynes
said. “The majority does converge on something that works well for their
patients and it takes a great deal of care, skill and artistry. But if you
ordered a socket from 20 different shops, you will receive 20 different

For thermoplastic material, technicians take great care
in pulling hot plastic over the positive model to ensure the thickness
retention in the distal area. This technique increases the robustness of the
thermoplastic socket. Check sockets are typically wrapped in fiberglass.
Commonly, prosthetic components are subjected to international standards —
ISO 10328. However, that standard does not include prosthetic sockets. In order
to conduct this study, Gerschutz and Haynes evaluated carbon and co-polymer
sockets using ISO 10328. In the study, impact energy was analyzed for the
effect of thickness, and diagnostic sockets were compared according to static
failure loads.

Gerschutz found that the stiffer, thicker material led
to a higher probability of brittle failure, which creates a balancing act
between strength and failure mode. This is an interesting finding because most
sockets are overbuilt to compensate for the many variances associated with

“The amount of variability surprised me a little
bit,” Haynes said. “I expected carbon sockets to be stronger than
they were. The general perception of carbon is that it is so incredibly strong
that it could never fail and if you look at the test results, they did pretty
well, but not as well as expected. Another material that was tested and was a
bit of an eye opener was the co-polymer socket. They are typically used for a
definitive socket. We always think of check sockets as strength challenged. But
the co-polymers we thought would be stronger as well.”

New technology advancements may be warranted, according
to the study. Gerschutz and Haynes are working on ways to test and compare
materials and practices used to make sockets. She will be presenting on a pilot
study, which focuses on static strength of check sockets, co-polymer sockets
and definitive laminated sockets at the American Orthotic and Prosthetic
Association meeting in Las Vegas in September.

Gerschutz has also worked on a larger study that
characterized the variances typically in clinical practice. She examined a
large number of sockets from a larger number of sources. That study has been
completed and is awaiting publication.

“We feel this information will provide a foundation to not only
evaluate the current processes being used to provide patients with sockets, but
also to discuss the appropriate levels of performance,” Gerschutz said.
“This will in turn, make it possible to evaluate future materials and
processes.” — by Anthony Calabro


I am glad that Ohio Willow Wood ran these tests and has
this documented because it is great to have these results down on paper.
However, the reason why we make these sockets so bulky is due to fear of
breakage and lawsuits.

I do agree that if you had 20 sockets go out, you will
receive 20 different sockets. But to play devil’s advocate, we also have
20 different patients and each one of them is going to need some mild
difference in design to their weight, height, scar tissue or other
considerations that come into play. This will result in different socket

We all strive for a certain thickness for these check
sockets. It is a balancing act to try to get them not too thick or not too
rigid, as well as not too thin so it breaks. I was surprised to see that the
carbon sockets do actually have a breaking point. We always feel as if they
will never break.

Ultimately, you have to go with what works for you. If
you are working with a large number of amputees and have a lot of volume,
CAD/CAM is probably the way to go because you are not wasting that time and
money. If you are a small shop and you are not working with a large number of
amputees, you will not see that benefit right away. Initially CAD/CAM is a big
financial investment. It’s best to go with what works for you, your
patient and your practice.

I do not feel this is black and white, but CAD/CAM is
probably the direction we all will be going in. It narrows that gap, so there
are not as many differences in socket designs, but we should keep in mind that
there are a wide variety of patients as well.

— Jeremy Crowell, BOCPO
fabrication manager, Hosmer Dorrance Corporation

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